[FEA] cubic spline velocity function for lateral variation

[Dirack]

🎉 Feature request

Is your feature request related to a problem? Please describe.

Use cubic spline functions to represent the velocity model variation in depth to allow models with lateral velocity variations.

Describe the solution you'd like

Use cubic spline functions of the velocity

Describe alternatives you've considered

You could split the velocity model in stripes and define a cubic spline velocity function of depth to each stripe.

Additional context

[Dirack]

Instead of using cubic spline, I've decided to use a 2D velocity grid to represent the velocity model and interpolate them using 2D eno interpolation to avoid discontinuities.